The invention concerns an X-ray analysis apparatus comprising                an X-ray source from which an X-ray beam is emitted,        at least one X-ray aperture which delimits the X-ray beam emitted by the X-ray source,        a sample onto which the X-ray beam, which is delimited by the at least one X-ray aperture, is directed, and        an X-ray detector for detecting X-ray radiation emanating from the sample,wherein the at least one X-ray aperture is disposed at a separation from the sample.        
An X-ray analysis apparatus of this type is disclosed by Youli Li et al., J. Appl. Cryst. (2008) 41, 1134-1139.
X-ray measurements, in particular, X-ray diffractometry and small angle X-ray scattering are used for qualitative and quantitative chemical analysis as well as for the structural analysis of samples in different fields of application.
In X-ray diffractometry, a high photon flux and high resolution are common requirements for X-ray analysis apparatus. For this purpose, the pinholes (X-ray apertures) are of decisive importance in addition to the X-ray source and the focusing or collimating X-ray optics.
The X-ray radiation that is incident on the apertures can be parasitically scattered which leads to an undesired increase in background radiation and therefore to a deterioration of the signal-to-noise ratio. In case of polycrystalline aperture material, parasitic radiation is generated on grains and grain boundaries and also by diffuse scattering on the surface roughness or total reflection at the aperture itself.
When in small angle X-ray scattering experiments, also called SAXS, an X-ray radiation wavelength λ is used, the resolution is given by the smallest possible measurable scattering angle θ or q vector (q=4π sin(θ)/λ). This means that in SAXS experiments as small a primary beam stop as possible should be used in order to guarantee optimum resolution. With small angles, parasitic scattering at the apertures is particularly strong and results in increased measurable intensities around the primary beam stop which are superimposed on the measurement signals of the sample, which can cause reduction of the resolution or even impairment of the correct counting behavior of the detector.
SAXS measurements moreover require X-ray radiation with small divergence. Parasitic scattering at the apertures increases the divergence, which has caused the manufacturers of X-ray analysis apparatus to position three pinhole apertures each of a defined size and at defined separations one after the other in the optical path of the SAXS analysis apparatus in order to guarantee an X-ray beam with defined low divergence. The disadvantage of this design with 3 pinhole apertures consists in the massive reduction of the photon flux and accompanying considerably longer measuring times and also in the large space requirement of these measuring devices.
Single crystal diffractometry experiments, also called SCD, also utilize pinhole apertures in addition to focussing X-ray optics due to the small sample sizes, the pinhole apertures being integrated in so-called apertures. The apertures are required for delimiting the X-ray beam to the predetermined sample size. When the X-ray beam is larger than the sample, the background noise increases e.g. due to scattered radiation of the sample holder and thereby deteriorates the signal-to-noise ratio. The parasitic scattering at the pinhole apertures further deteriorates the signal-to-noise ratio. In SCD measurements, primary beam stops are used which should be selected to be as small as possible in order to only minimally delimit the q space to be examined and therefore the resolution. Due to the parasitic scattering at the pinhole apertures, a correspondingly larger primary beam stop must be used in consequence of which the resolution is correspondingly smaller. If the resolution around the primary beam is decisive, e.g. in protein crystallography, only very small pinhole apertures can be integrated when the primary beam stop size remains the same, which, in turn, greatly reduces the photon flux and correspondingly prolongs the measuring time.
In μ diffraction applications, also called μ-XRD, X-ray diffraction is supposed to occur only selectively at very small surfaces or volumes of samples. Towards this end, the X-ray cross-section must be limited to a diameter of typically 10-100 μm, which also requires pinhole apertures in addition to X-ray optics. The parasitic scattering at conventional polycrystalline pinhole apertures thereby increases the surface illuminated on the sample. For this reason, the pinhole aperture size must be further reduced, which results in longer measurement times. Moreover, measurements have shown that the polycrystalline structure of the aperture becomes visible on the detector in the form of Debye-Scherrer rings, which are superimposed on the measurement signal of the sample.
Conventional X-ray apertures consist of polycrystalline metals having very good X-ray radiation absorption properties, such as tungsten, tantalum, iridium, brass, titanium, molybdenum or platinum, wherein different opening sizes and opening shapes are available. One disadvantageous property of these pinholes is the relatively strong parasitic scattering of X-ray radiation.
In Rev. Sci. Instrum. 66, 1354 (1995), Gehrke et al. have proposed an X-ray aperture for synchrotron applications which reduces parasitic scattering. The aperture comprises four motorized blades which are movably disposed opposite to each other such that a rectangular X-ray cross-sectional area is produced which can be adjusted in size. The blades consist of a material with good X-ray radiation absorption properties and their end faces comprise single crystal silicon layers. In order to prevent total reflection and therefore parasitic scattering at the end faces of the blades, the blades are tilted by a tilt angle which is larger than the angle of total reflection.
A similar principle has also been disclosed for hybrid metal single crystal blades, cf. Li et al.; Appl. Cryst. 41, 1134 (2008). Instead of tilting the blades in order to prevent total reflection, chamfered metal bases, e.g. of tungsten or brass, are used at each end face, onto each of which one rectangular single crystal substrate is glued which is polished on one side. In this fashion, the X-ray beam is mainly defined by the edges of the oppositely disposed single crystal substrates, which prevents parasitic scattering due to total reflection and also due to scattering on grain boundaries.
In document FR 2 955 391 A1, hybrid metal single crystal blades are integrated in an X-ray analysis apparatus, which is especially designed for SAXS measurements and is supposed to have a compact size in comparison with other commercially available SAXS machines.
X-ray apertures on the basis of these conventional hybrid metal single crystal blades do, in fact, enable two-dimensional beam shaping with very little parasitic scattering. However, they are difficult to produce. Manual work is constantly required. The X-ray apertures are also difficult to adjust, in particular, with respect to relative positioning of the blades. Moreover, they enable only a rectangular beam cross-section and, with respect to the resolution, at best a square beam cross-section. The minimum primary beam stop size is given by the separation between the center and the outermost point of the beam cross-sectional area. This means that a round beam cross-section permits a minimum primary beam stop size and therefore maximum resolution. In comparison therewith, the resolution of a square beam cross-section and therefore also the resolution of the systems with four hybrid blades are deteriorated by at least 41.4% (corresponds to √2).
Document JP 2002 250 704 A discloses a mask that is applied to a sample in order to select part of the sample for exposure to X-ray radiation. The mask has a funnel-shaped opening, wherein the narrower part of the opening faces the sample. The mask is produced from a single crystal material.
It is the underlying purpose of the present invention to present an X-ray analysis apparatus, in which the influence of parasitic scattered radiation on X-ray measurements is reduced with little expense.